"Of late, Turkey more often makes unfounded accusations, in particular, that Russia's military aircraft allegedly violate the airspace," he said. "Most often, this misinformation launches anti-Russian campaigns in the Turkish mass media or becomes their part."

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Russian airstrikes in Syria, which borders Turkey, have been conducted to bolster President Bashar al-Assad's regime. Russia has said it's coordinating with the Syrian regime to target ISIS and other terrorists.

'Quietly spoken professional'

Richard Moore, Karlov's British counterpart in Turkey, tweeted of his shock. He described the ambassador a "quietly spoken, hospitable professional."

Sir Tony Brenton, who was Britain's ambassador to Moscow from 2004 to 2008, did not know Karlov. However, he told CNN: "As a former ambassador myself, you always know you're a symbol and also a target. Something like this is particularly horrifying when it happens. My first thought was sympathy for his family and colleagues."